With recent improvement and advancement of electronics technology for mobile PCs, mobile phones, personal digital assistants (PDAs), etc., secondary batteries and the like, which can be repeatedly charged and discharged, are widely used as power storage devices for these electronic apparatuses.
Of these secondary batteries, a lithium-ion secondary battery, which includes a positive electrode prepared by using a lithium-containing transition metal oxide such as lithium manganese oxide or lithium cobalt oxide as an electrode active material and a negative electrode prepared by using a carbonaceous material capable of lithium ion insertion/desorption, is in widespread use.
However, this lithium-ion secondary battery is a power storage device that generates electric energy through an electrochemical reaction, and the aforementioned lithium-ion battery using a lithium-containing transition metal oxide in a positive electrode has a critical problem of a lower power density because of its lower electrochemical reaction rate. The lithium-ion secondary battery still has room for improvement in capacity density per unit weight because of a large specific gravity of the lithium-containing transition metal oxide.
There is also known a nonaqueous electrolyte secondary battery in which an electrically conductive polymer, such as a polyaniline containing a dopant, is used as a positive electrode active material to improve the power density (see PLT1). In general, however, since the secondary battery employing the electrically conductive polymer as the positive electrode active material is of an anion migration type in which the polymer of the positive electrode is doped with an anion in a charge period and dedoped with the anion in a discharge period, the secondary battery is a so-called reserve type secondary battery in which an ion concentration in an electrolyte solution varies during charge/discharge. Accordingly, the nonaqueous electrolyte secondary battery employing the electrically conductive polymer as the positive electrode active material basically requires a large amount of an electrolyte solution, and therefore the nonaqueous electrolyte secondary battery has a problem that it is impossible to contribute to the size reduction of the battery.
In order to solve such a problem, a secondary battery is also proposed that is of a cation migration type and substantially free from change in the ion concentration in the electrolyte solution by using, for the positive electrode, an electrically conductive polymer containing a polymer anion such as polyvinyl sulfonate as a dopant (see PLT 2).